The proposed tunnels would run under Salmon Bay, and would not be covered by the $54 billion included under ST3. Because of that, the project would require third-party money in order to properly fund it.

Past estimates have pegged the tunnels as costing $350 million more than plans to instead lay tracks over a draw bridge. The bridge would be taller than the Ballard Bridge, and open two to four times a day for marine traffic.

Also making it into final consideration was a high fixed bridge across Salmon Bay roughly as tall as the Aurora Bridge. That would cost around $100 million more than a draw bridge, but still $250 million less than tunnels.

Supporters of the tunnels claim that it would be less disruptive to neighborhoods in the long run, and more aesthetically pleasing, ultimately making it worth the added cost.

“The tunnel is definitely what would work best for everyone,” Port of Seattle Commission President Stephanie Bowan told KIRO 7 TV last month.

Opponents, though, have argued that the time and money needed for tunnels would delay the expansion of light rail to the North Sound area. With that in mind, all three Sound Transit Board members working for Snohomish County voted against the project, joined by two officials from Pierce County.

Snohomish County Executive Dave Somers noted in Thursday’s board meeting that if chosen, the tunnels should be paid for by the City of Seattle, and that county officials shouldn’t be on the hook for the increased costs.

The total expansion plan adds almost five miles of light rail between West Seattle and downtown Seattle, with five stations between the stadium area and the Alaska Junction. The stations are slated to open in 2030.

By 2035, another seven miles of light rail will run between Ballard to downtown, which would include nine stations between the Chinatown-International District and Ballard.

For now, the Sound Transit board will move forward to study four total options: Two with tunnels, separated largely by the placement of stations, a draw bridge, and a high fixed bridge. Sound Transit will vote on its final decision in 2020.